Electric machines, motors and generators require some means to control the output current, speed of rotation, excess heat generation and accommodate failure modes, such as those due to short circuit conditions and the like.
For example the inventor's U.S. Pat. No. 6,313,560 relates to electric machines that are thermally protected from damage resulting from internal faults, such as high short circuit currents, using the Curie point properties of low Curie point materials, such as ferrite in constructing the machine. Heat generated by an internal fault, such as a short circuit, heats the material above its Curie point, i.e.: where magnetic flux circulation is impeded and the machine is shut down. Careful selection of a material having a Curie point below the maximum permissible operating temperature for the machine thus permits the low Curie point material to act as a sort of thermal fuse for the device.
Permanent magnet electric motors and generators usually include a rotor of magnetic material such as Samarium-Cobalt. In close proximity to the rotor, electric windings on a stator carry current that generates a magnetic field, in a motor, or that carry current induced by a rotating magnetic field generated by the rotor, in a generator.
As is well known, the motor/generator arrangement can be reversed such that an internal stator is housed inside an external rotor. It will be understood therefore that the present invention and description of prior art equally applies to both common electrical machine configurations with internal and external rotors.
In aircraft engine starter-generator applications, an electric machine (generally preferably connected directly to a main spool of the gas turbine) replaces a conventional auxiliary gearbox. Disadvantageously, however, electric machines coupled to aircraft turbine engines can potentially generate extreme power limited only by the power of the turbine engine driving the rotor of the machine. Unabated, generation of such electric power can result in extreme heat, particularly in the stator windings, that may cause the motor to melt and potentially burn. This is clearly undesirable, especially in aircraft.
Current provided by the machine to external electrical equipment may be limited by fusing arrangements, but such fusing arrangements provide no protection to the machine itself. The device described in the inventor's U.S. Pat. No. 6,313,560, thermally protects the motor/generator itself from damage resulting from internal faults.
When a low Curie point material, such as ferrite is incorporated into a machine, the time required for the ferrite in the low Curie point material in the machine to reach the Curie temperature during the internal fault event, is related to the ratio of thermal capacity to thermal conductivity and the temperature differences between the stator material and the surroundings. This has implications for response times, especially in larger machines. Accordingly, opportunities for improvements in design are available.
It is an object of the present invention to improve the reaction time of a low Curie point internal fault protection system.
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.